Method and apparatus for thickening a slurry using a compression plate

ABSTRACT

Method and drum filter for thickening of slurry and for treatment of the slurry with liquid. The slurry to be thickened is passed onto the filter face of a drum (3) revolving in a basin (3), and the treatment liquid is passed through the pulp web formed on the drum. The consistency of the pulp web is increased by pressing a compression plate (10) against the pulp web. The compression force of the compression plate (10) is produced by means of the pressure of a liquid and/or gas passed to the convex side of the plate. The compression force can be adjusted by adjusting the pressure prevailing in the gas space outside the drum (3).

The present invention concerns a method for thickening of slurry and forits treatment with liquid, whereat the slurry to be thickened is passedonto the filter face of a drum revolving in a basin and the treatmentliquid is passed through the pulp web formed on the drum and whereat theconsistency of the pulp web is increased by pressing a compression plateagainst the pulp web. The invention also concerns a drum filter forthickening of slurry and for its treatment with liquid, which saidfilter comprises a basin, a filter drum revolving in the basin, a boxfitted above the drum, means for producing a positive pressure in thespace formed by the basin and by the box, means for feeding slurry ontothe outer face of the drum, means for passing the treatment liquid ontothe pulp web placed on the drum, means for detaching the thickened pulpweb from the drum, as well as means for removing the filtrate frominside the drum, whereat the filter is provided with a flexible plateplaced after the pulp-feed point at a distance from the outer face ofthe drum.

For the thickening and washing of fibrous slurries, in particular ofcellulosic fibrous slurry or pulp slurry, filters are used in which thefibrous slurry is thickened on a revolving drum. The fibrous slurry ispassed into the feed box of the filter as diluted to an appropriateconsistency. The fibrous slurry is thickened by filtration by means of adifference in pressure onto the face of the filter drum, which saidface, as the drum revolves, carries the thickened slurry further.Washing water is passed onto the pulp web placed on the drum face, saidwater passing through the pulp web into the drum while displacingchemicals contained in the pulp.

When attempts are made to increase the capacity of a washing drum byincreasing the speed of rotation and the consistency of the pulp web, aproblem that is encountered is how to provide a uniform pulp web. If thepulp web becomes uneven, the washing result cannot be made good.

In the U.S. Pat. 4,085,003 a washing drum is described wherein acompression plate to be pressed against the pulp web is employed. Bymeans of the plate, the consistency of the pulp is increased and theuniformity of the pulp web is improved. The plate is provided withrigidifying ribs, and it is pressed against the pulp web by means ofactuator members provided at the outer end of the plate. It is problemof this prior-art device that occasionally the washing liquid startsflowing to below the compression plate. This comes from the fact thatthe compression plate is rigid and is not always placed tightly againstthe pulp web, especially if the pulp web is uneven.

The object of the present invention is to provide a drum filter whereinthe operation of the compression plate is arranged in a simpler and moreefficient way as compared with prior art. The method in accordance withthe invention is characterized in that the compression force of thecompression plate is produced by means of a pressure effective at theconvex side of the plate. The drum filter in accordance with theinvention is characterized in that the flexible plate can be pressedtowards the drum by means of a pressure effective at the convex side ofthe plate.

By means of a drum filter in accordance with the invention, theconsistency of the pulp web can be increased highly efficiently rightbefore the washing, at the same time as a uniform pulp web is provided.This is important in order that a good washing result could be obtained.When the compression force of the plate is produced by means ofpressure, no particular actuator member is required. Since a resilientcompression plate is also pressed well against an uneven pulp web, thewashing liquid has not access to below the plate at the trailing edge ofthe compression plate.

DESCRIPTION OF THE DRAWINGS

The invention and its details will be described in in the following withreference to the accompanying drawings, wherein

FIG. 1 is a cross-sectional view of one embodiment of the drum filter inaccordance with the invention, and

FIG. 2 of another embodiment.

DETAILED DESCRIPTION OF THE INVENTION

The drum filter comprises a basin 1 and, above the basin, a hood 2,which together form a closed space. Into the basin, a revolving drum 3is fitted, which is provided with a filtering wire face. The filter isprovided with a blower 4 so as to maintain a positive pressure in thespace defined by the basin and the hood and placed outside the drum. Thedirection of rotation of the drum is clockwise in the drawing.

In the lower part of the filter, there is a feed box 5 for feeding thepulp onto the drum face. By the effect of the difference in pressure,the filtrate water runs out of the web into the interior of the drum.The drum-side wall of the feed box is sealed by means of a lip seal 6against the moving face of the drum. The other wall 7 of the feed boxfollows the shape of the drum at a distance, whereby a pulp-webformation zone 8 remains between the drum and the wall 7.

The washing water of the first washing stage is passed into the space 9above the feed box, from where it is pressed through the pulp web andthe drum into the drum. At the upper edge of the drum-side wall of thewashing-water space 9, as an extension of the wall 7, a flexible,resilient plate 10 is attached over the entire length of the drum. Theupper edge of the plate 10 can move freely towards the drum, and thepressure of the washing water effective at the convex side of the platepresses the plate against the pulp web.

The washing water of the second washing zone is passed onto the drumface out of the jet pipes 11.

Between the flexible plate 10 and the drum, after the pulp-web formationzone 8, a thickening zone 12 is formed, wherein the plate compresses thepulp and removes liquid out of the pulp. The plate is not provided withany rigidifying members, but it is placed freely against the pulp webdepending on the pressure effective at the other side of the plate.

The pulp web 13 continues its run further on the drum face through thesecond washing zone, and by the effect of the pressure outside the drumwater is separated from the pulp web into the drum. At the point 14 thepulp web is detached from the drum face in a way in itself known.

The filtrate water is removed out of the drum.

In the figure, the level of the washing water is denoted with thereference numeral 15, and the level of the filtrate water with thereference numeral 16.

Besides by means of the hydrostatic pressure of the washing water, thecompression force of the compression plates can also be affected bymeans of the air pressure effective inside the hood. By adjusting theair pressure, it is also possible to adjust the compression force.

It is also possible to run the washing stages so that no water from thefirst stage enters into the space 9, but all the washing water isintroduced into the second washing stage out of the jet pipes 11. Insuch a case, it is only the pressure in the gas space in the hood thatacts upon the flexible plate 10.

The material of the plate 10 is most appropriately a synthetic material.It is important that the friction between the plate and the pulp webshould be as low as possible in order that the pulp web should followthe moving face of the drum, whose friction is higher than the frictionof the plate.

In tests that have been carried out, it has been possible to increasethe consistency of the pulp by means of the compression plate from aconsistency of 5 to 8% at the initial end of the plate to a consistencyof 13 to 16% at the final end of the plate, in other words, one half ofthe water present in the pulp was removed within the thickening zone.

FIG. 2 shows an embodiment of the invention provided with two separatethickening zones 12 and 12' and with two flexible plates 10 and 10'. Thewashing water of the second washing zone is passed into the secondwashing-water space 9' placed above the first washing-water space 9.

The invention is not confined to the embodiments described above alone,but it may show variation in different ways within the scope of thepatent claims. The invention can also be applied in single-stagewashing, and so also in washing comprising a higher number of washings.Besides to the washing of pulp, it can also be applied to treatment ofpulp with other liquids. If necessary, it is possible to fitliquid-collecting basins inside the drum in a way in itself known, inwhich case the filtrates from the different washing stages can becollected separately and, if necessary, be passed, e.g., to thepreceding washing stage.

It is possible to construct a similar compression plate also for asuction washer, in which case the hood is not pressurized but, on thecontrary, there is negative pressure inside the drum.

What is claimed is:
 1. A method for thickening of slurry and for itstreatment with liquid, comprising the steps of:passing the slurry to bethickened onto a filter face of a drum revolving in a basin covered by ahood so as to form a space surrounding the drum; passing the treatmentliquid through the pulp web formed on the drum; and increasing theconsistency of the pulp web by pressing a flexible compression plateagainst the pulp web by means of a pressure acting upon the face of theflexible compression plate facing away from the drum, said pressurearising solely from at least one of hydrostatic pressure of thetreatment liquid and overpressure of a gas within said space.
 2. Themethod as claimed in claim 1, wherein the compression force of thecompression plate is adjusted by adjusting the pressure of said gasprevailing in the space surrounding the drum.
 3. A drum filter forthickening of slurry and for its treatment with a liquid, said filtercomprising:a basin and a hood together forming a space; a revolvingfilter drum mounted in said space; means for producing a difference inpressure between the space outside the drum and the space inside thedrum; means for feeding pulp web slurry onto the outer face of the drum;means for passing the treatment liquid onto the pulp web placed on thedrum; means for detaching the pulp web from the drum; means for removingthe filtered liquid from inside the drum; a flexible compression plateplaced after the pulp-feed point at a distance from the outer face ofthe drum, which flexible compression plate is adapted to be pressedtowards the drum by means of a pressure effective at the side of theplate that is facing away from the drum and arising solely from at leastone of hydrostatic pressure of the treatment liquid and overpressure ofa gas within said space, said flexible compression plate being locatedso that said pressure acts upon the face of the plate facing away fromthe drum.
 4. The drum filter as claimed in claim 3, wherein the edge ofthe flexible plate (8) that is placed away from the pulp-feed point canplace itself in an appropriate position.
 5. The drum filter as claimedin claim 3, wherein several flexible plates have been fitted on thecircumference of the drum one after the other.
 6. The drum filter asclaimed in claim 4, wherein several flexible plates have been fitted onthe circumference of the drum one after the other.